Thermal stress is evaluated using insulation resistance trends, DGA (for oil-filled units), visual inspection for discoloration or carbonization, and sometimes SFRA to check for mechanical deformation caused by overheating. Winding resistance measurements can reveal damage from local hot spots. Temperature sensors, if installed, provide historical hot-spot data. After repair, controlled load tests and monitoring of temperature rise validate that thermal behavior is within design limits before full service is resumed in the DC system.